Effect of Welding Materials on Microstructure and High-Temperature Properties of UMCo50 Overlay Layers

Literature Overview

The study by Xiong Jiang, Xin Zhengqiu, Li Wenchao, Xue Haitao, and Tang Qiang (published in the journal Hot Working Technology, 2024) investigates the influence of different welding consumables on the microstructure and elevated-temperature mechanical performance of UMCo50-based overlay layers. UMCo50 is a cobalt-based hardfacing alloy widely employed in high-temperature, high-wear, and corrosive service environments such as hot gas path components, valve seats, and petrochemical processing equipment. The research is jointly conducted by CNOOC Huizhou Petrochemical Co., Ltd., Beijing Hanghua Energy Saving and Environmental Protection Technology Co., Ltd., and Hebei University of Technology, reflecting a strong industry-academia collaboration.

Core Technical Points

Material System and Process Context

UMCo50 is a Co-Cr-W-C type hardfacing alloy whose wear resistance and thermal stability derive from the precipitation of hard carbides (primarily M7C3 and M23C6 type) within a cobalt solid solution matrix. The microstructure and carbide distribution are critically dependent on the welding process parameters, filler metal composition, and dilution from the substrate. The study focuses on how varying welding materials (filler wire or flux composition) affect:

• Carbide morphology, size, and distribution
• Matrix phase composition (gamma phase, sigma phase, and other intermetallics)
• High-temperature hardness retention
• Oxidation resistance at elevated temperatures
• Thermal fatigue crack resistance

Microstructural Evolution

The microstructure of UMCo50 overlay layers is typically characterized by:

The researchers found that the choice of welding material significantly influences the carbon activity during solidification. Higher carbon activity in the melt leads to more complete carbide precipitation, while lower carbon activity can result in carbon-depleted zones adjacent to carbides, reducing the overall hardness of the overlay.

High-Temperature Performance

The key performance indicators evaluated at elevated temperatures include:

The study demonstrates that optimized welding materials yield overlay layers with superior high-temperature hardness retention. This is attributed to:

1. Finer and more uniformly distributed carbides that resist coarsening at elevated temperatures
2. Reduced sigma phase content, which embrittles the matrix at high temperatures
3. Better control of dilution, ensuring the alloy composition remains within the optimal range for thermal stability

Process Analysis and Engineering Implications

Welding Process Selection

UMCo50 overlay is commonly applied using:

• Submerged arc welding (SAW) for thick overlay layers on large components
• Gas tungsten arc welding (GTAW/TIG) for thin, precise overlay layers
• Flux-cored arc welding (FCAW) for production efficiency
• Plasma transferred arc (PTA) for low-dilution, high-quality surface layers

The selection of welding material must be matched with the welding process. For example, FCAW with UMCo50 flux-cored wire provides consistent composition control, while SAW with UMCo50 electrode combined with specific flux formulations allows tuning of the carbon and alloy content in the final overlay.

Dilution Control

Dilution from the substrate is a critical factor in cobalt-based overlay welding. The study emphasizes that:

• Multi-pass overlay strategies should be employed to reduce dilution in the final layers
• Preheating to 200–300 °C is recommended to reduce thermal stresses and minimize cracking
• Interpass temperature control (150–250 °C) is essential to prevent excessive grain growth and sigma phase formation
• The first pass typically experiences 30–50% dilution, while subsequent passes may have 5–15% dilution

Common Defects and Countermeasures

Study Insights and Reflections

This research is particularly valuable for petrochemical applications where UMCo50 overlays are used on hot gas path components, valve trim, and catalyst support structures. The findings reinforce a fundamental principle in overlay welding: the welding material is not merely a filler—it is the primary determinant of the overlay's metallurgical quality and service performance. Engineers must carefully match the filler metal composition with the specific service conditions (temperature, wear mechanism, corrosive environment) rather than relying solely on nominal alloy designations.

The industry-academia collaboration model demonstrated in this study is commendable. The involvement of CNOOC Huizhou Petrochemical provides real-world service data and failure analysis context, while the academic institutions contribute fundamental metallurgical understanding. This approach ensures that research outcomes are directly applicable to industrial challenges.

From a practical standpoint, the study reinforces the importance of qualification testing for cobalt-based overlay systems. Unlike carbon steel or stainless steel overlays, cobalt alloys have complex phase equilibria that are highly sensitive to minor compositional variations. Engineers should not assume that all UMCo50 consumables from different manufacturers will produce equivalent results; thorough qualification testing in accordance with applicable codes (such as ASME IX or equivalent national standards) is essential.

The study also highlights an area for further investigation: the long-term creep behavior of UMCo50 overlays at temperatures above 800 °C. While the high-temperature hardness and oxidation resistance are well characterized, the creep resistance of these overlays under sustained load remains insufficiently documented in the literature. Future research should address this gap, particularly for applications in turbine hot sections and high-pressure hydrogenation reactors where creep-fatigue interaction is a concern.